GaN-based semiconducting materials including GaN, InGaN, AlGaN, AlInGaN and the like have greater bandgap energy than GaAs-based materials and the like. Moreover, GaN-based semiconducting materials have a high heat resistance, thereby being excellent in a high-temperature operation. Therefore, development research in various kinds of devices which take advantage of the above-mentioned characteristics of GaN-based semiconducting materials, has been proceeded by using GaN, especially.
An example of a GaN-based HEMT configuration is shown in FIG. 33.
In this HEMT configuration, there is formed on a semi-insulating substrate 91, such as a sapphire substrate, a heterojunction structure in which a buffer layer 92 made of for example GaN, an undoped GaN layer 93, and for example an undoped AlGaN layer 94, which is much thinner than the undoped GaN layer 93, are stacked in that order. On the undoped AlGaN layer 94, there are disposed two n-AlGaN contact layers 95 doped with for example Si of high concentration, which is an n-type impurity. A source electrode S and a drain electrode D are arranged on their respective contact layers 95. Moreover, a gate electrode G is formed onto the undoped AlGaN layer 94 that spreads between the source electrode S and the drain electrode D.
The contact layers 95 are provided for the purpose of upgrading the ohmic junction characteristic between the source(S) and drain(D) electrodes and the semiconductor, so that the contact layers 95 do not have to be provided if ohmic junction can be obtained without them.
FIG. 34 is an enlarged view of a portion P1 encircled by a broken line in FIG. 33 to clearly show the position in which two-dimensional electron gas 96 is produced.
In this HEMT configuration, during the operation of the HEMT, the undoped AlGaN layer 94 functions as an electron supply layer and supplies electrons to the undoped GaN layer 93. Once the source electrode S and the drain electrode D are operated, the electrons supplied to the undoped GaN layer 93 travel through the two-dimensional electron gas 96 to the drain electrode D. Accordingly, the undoped GaN layer 93 serves as a channel layer.
In the case of the above-mentioned HEMT configuration, there generate the two-dimensional electron gas in the whole area of the heterojunction interface expanding from the source electrode S to the drain electrode D. In the HEMT configuration, even if a gate voltage is brought to 0 V to make the gate being off, a pinch-off voltage is not 0 V due to the presence of carriers in the channel layer. Consequently, there is provided a normally-on FET in which drain current keeps flowing.
Therefore, in order to prevent the drain current, it is required that the gate electrode be constantly applied with a gate bias voltage that is equal to or more than a gate threshold value.
In cases where the HEMT configuration is employed as a switching device for a power source, however, it is unfavorable in terms of electricity consumption to keep applying the given voltage to the gate electrode in order to turn the switch off.
Considering the aforementioned, if it is possible to achieve a normally-off FET in which the drain current does not flow at opening the gate with the above-described HEMT configuration, its industrial value is enormous.
An object of the present invention consists in providing a GaN-based FET of a normally-off type, which has a small ON resistance and is capable of a large-current operation.